Boulder Opal

The Australian boulder opal is typically considered the second most valuable kind, after the black opal. Here are a few of its characteristics:
Appearance

The boulder opal is identified by its characteristic solid brown ironstone layer on its back. These opals form in thin colourful veins that run through the cracks and voids within boulders. Opal cutters often need to retain the ironstone layer to form a full sized stone. The stone's innate value is borne out of the way in which it diffracts light exhibiting a brilliant 'play of colour'.

Boulder Opal

The ironstone layer that remains on a boulder opal often makes the stone appear like a doublet on first glance, but unlike a doublet, the boulder opal is completely natural. It is this ironstone layer that gives the stone its relatively dark body tone. As a consequence of this, like the black opal, the boulder opal exhibits vibrant and easily distinguishable colours when held at different angles.
In many cases, the ironstone layer remains visible on the surface of the stone giving it a spotted appearance. If the ironstone layer is exposed on the surface of the stone, it may adversely affect its value. Spotted stones affect the 'play of colour' that boulder opals display. Alternatively, the most valuable kind of boulder opals are those which are 'clean faced'. When an entire bar of colour is visible on the stone, the stone can fetch a significant price.

A boulder opal is also valued based on the thickness of its bar of colour. A thick bar means that it can be cut with a high cabochoned surface and is likely to be extremely rare. Most boulder opals display a flat surface. Unlike other forms of opals, the boulder opal is often cut in freeform to increase the size of the stone.

Value

Unlike other opal stones, it is not feasible to value the boulder opal by carat. This is primarily because of the large ironstone content found within the stone. However, it is seen that a superior boulder opal can fetch a price anywhere up to AUD $3,000. As mentioned earlier, a clean faced stone is likely to be more valuable than one that has ironstone spots on its surface.

Origin

While other kinds of opals may be found around Australia, boulder opals are found primarily within the state of Queensland. Quilpie and Winton are two of the more popular opal fields that are rich in boulder opals.